I think Prapai thinks Sky is playing hard to get, and won't stop without a more definitive rejection. That being said, him changing his number is crossing that line. It reminds me of how Payu crossed the line when Rain told him to let go of him in the first episode.
We should remember that Prapai doesn't know Sky's past so it appears to us as more insensitive when he continues to pursue him. I'm now hoping Prapai finds out who Sky's ex was and how bad he hurt him.